The present invention relates to partially translucent, white films having a metallized surface. More particularly, the invention relates to flexible sheet articles having a highly reflective surface, and have the ability to partially and controlledly transmit diffuse light therethrough.
In the art of color proofing, color separations made from an original image should represent a faithful reproduction of tonal values, and the proofing methods used to make representations of these halftone separations should reproduce the quality seen in the photomechanically produced originals. However, when these high quality color proofing separations are compared to the output of a high quality printing press, the results do not always correspond. This lack of correspondence is the result of the printing press producing a halftone dot size on paper that is different from the halftone dots on the printing plate, or color separation itself. The observable result is what is called in the trade as "dot gain". It is produced by the spreading out of ink on the printed sheet to cover an area different than is represented by the printing plate or color proofing originals. These problems have been recognized in the printing industry and certain techniques have been developed to adjust for these variations. Color proofing systems are well known and are broadly disclosed in the prior art. The techniques of progressing from an original colored artwork through a series of color separations, and the use of these separations to generate color proofing separations are well known in the art.
In the art of printing, it is desirable to produce a multi-color proof to assist a printer in correcting a set of color separations which will be used in exposing a series of expensive lithographic printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired half tone or line image. Visual examination of a color proof should reveal the color rendition to be expected from a press using the color separations, and any defects on the separations which might need to be altered before making the printing plates.
Color proofs for multicolored printing can be made by using a printing press which is known as a proof press. This requires that all of the actual printing steps be formed and this conventional method of color proofing is costly and time consuming. Alternate color proofing methods have therefore been developed to simulate the quality of press proofs. There are two general types of photoimaging methods, namely the overlay type and the single sheet type. In the overlay color proofing system, an independent transparent plastic support is used for producing a colored image of each color separation film. A number of such supports carrying the various colored images of cyan, yellow, magenta and black are then superimposed upon each other and placed on a white sheet to produce a color proof. Examples of this approach are described in U.S. Pat. Nos. 3,136,637; 3,211,553; and 3,326,682. In the single sheet color proofing method, a color proof is prepared by successively producing images of the colors from different color separation films onto a single receiver sheet. This is done by using a single opaque support and by applying toners, photosensitive solutions, or coatings of photosensitive materials of corresponding colors to the opaque support in succession. Examples of this approach are described in U.S. Pat. Nos. 3,671,236; 4,260,673; 4,366,223; 4,650,738; 4,656,114; and 4,659,642. An advantage of the single sheet type of color proof is that the color is not influenced by superimposed plastic supports. This method more closely resembles actual printing and eliminates the color distortion inherent in the overlay system. Various processes for producing copies of an image embodying photopolymerization and thermal transfer techniques are known as shown in U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025; 3,481,736; and 3,607,264. In these processes, a photopolymerizable layer coated on a support is imagewise exposed through a color separation. The surface of the exposed layer is then pressed into contact with the image receptor sheet and the composite is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, and the image areas transferred to the image receptor. U.S. Pat. No. 3,721,557 shows a method of transferring colored images to a receptor sheet. An image carrying support is pressed against a suitable adhesive coated receptor member and subsequently, the carrier support sheet is stripped to accomplish the transfer of the image. A fresh layer of adhesive is applied to the receptor for each subsequent transfer. U.S. Pat. No. 4,596,757 provides a method for transferring images or solid colors which are subsequently imaged. The photosensitive material comprises a carrier support having sequentially disposed thereon a release layer; a colored, photopolymerizable layer; and an adhesive layer. The material can undergo exposure, lamination to a temporary support, wet development, and then lamination to a receptor sheet.
Images can also be produced by a peel apart method. Peel apart color proofing systems are known to the skilled artisan per se. Peel apart methods are described in U.S. Pat. No. 4,489,154, where a photosensitive layer is disposed between two self supporting sheets. Upon exposure, there is an imagewise differential in adhesion of the exposed and unexposed portions of the photosensitive image to one of the two sheets forming the photosensitive element.
It has been found that to various extents, the substrate on which the final image appears, contributes to an increase in apparent dot gain on the proof. This is disadvantageous since it contributes to the production of an image which is not a faithful reproduction of the original image. The prior art has recognized this problem and has sought to achieve a receiver base for image transfer which does not increase apparent dot gain. To this end, U.S. Pat. No. 4,579,810 discloses a multicolor proof having a pearlescent support. It has now been found that dot gain can be controlled using a partially translucent support which is white on one side and has a spectrally reflective, mirror-like metallic surface on the opposite side. The white side allows an accurate reflective background base from which to view the colors of the image. White backgrounds, while themselves well known in the art are also transmissive to part of the light cast upon them. Since the background materials, usually white polyester films, have a certain thickness, they tend to reflect part of this transmitted light back through the image in a diffuse fashion and cause a shadow which is viewed as a gain in dot size. This invention solves this problem by coating the opposite side of the support with a mirror-like metallic coating. This causes the inevitable reflections to be much more coherent rather than diffuse, thereby substantially reducing shadow and hence dot gain. In the first instance one would assume that the same result could be attained by employing a base which is completely opaque. However, it has been found that in order to correctly register sequential image separated halftone images as is necessary in color proofing, the receiver support must have a certain degree of translucency. The present invention therefore provides an improved white receiver base for color proofing images which possesses the requisite degree of translucency for image registration and yet has substantially reduced apparent dot gain.